Soy (Glycine max) has long been the most widely used plant-based protein source, widely adopted as an alternative to animal-derived foods. However, as global demand for plant proteins continues to rise, reliance on a single legume crop, such as soy, causes sustainability and supply challenges. Consequently, research attention has shifted towards identifying alternative, underutilised legumes with favourable nutritional and environmental benefits for human consumption. Lupinus angustifolius L. (Australian sweet lupin or narrow-leafed lupin) has emerged as a promising crop due to its high protein and dietary fibre content, absence of trypsin inhibitors, and non-GMO status, compared to soybeans. From an environmental viewpoint, lupins are well-adapted to acidic and sandy soils, requiring minimal fertiliser input, and offer a resilient and sustainable option for cultivation in resource-limited agricultural systems. Protein isolates were extracted from dehulled seed flour of five L. angustifolius genotypes grown at an experimental site in Merredin, Western Australia, using alkaline extraction and isoelectric precipitation. The resulting isolates were subjected to a comparative evaluation of their thermal, structural, and rheological characteristics, with commercial soy protein isolate (SPI) serving as a reference material. Statistical analysis was conducted using IBM SPSS Statistics (Version 30, 2024). One-way analysis of variance (ANOVA) with Tukey’s HSD post hoc test was performed to determine significant differences between groups at p ≤ 0.05. The DSC thermograms, FTIR spectra, and rheological curves were processed using TRIOS (v5.3, 2023), OPUS (v7.0, 2019) and RheoCompass™ (v1.32, 2023) software, respectively. Fourier Transform Infrared (FTIR) analysis indicated that β-sheets were the most abundant secondary protein structure in lupin protein isolates (LPI), followed by α-helices, with a comparable pattern observed in SPI. Differential Scanning Calorimetry (DSC) revealed two distinct denaturation transitions in the LPI, with peak denaturation temperatures (Td) observed between 84-86°C and 96-98°C, corresponding to the thermal unfolding of the β-conglutin and α-conglutin fractions, respectively. In contrast, the first two thermal transition peaks, corresponding to β-conglycinin and glycinin, respectively, were absent in the soy protein isolate, possibly due to protein denaturation caused by the extraction and post-extraction conditions or the high temperatures employed during spray or drum drying in the commercial production of soy protein isolate. The protein network of LPI lacked a well-defined structure and exhibited a slightly porous, irregular morphology. The microstructure of lupin proteins was less interconnected than soy, which had a more compact and continuous protein network. Rheological analysis showed that LPI formed weaker and more easily deformable gels, as evidenced by their lower complex viscosity (η*), storage modulus (G′), and loss modulus (G″), along with a higher loss factor (tan δ) relative to SPI. Despite forming comparatively weaker gels than soy proteins, lupin proteins demonstrated higher denaturation temperatures and greater thermal resilience, indicating their potential suitability for incorporation into thermally processed, high-protein food systems. Overall, these findings highlight both the challenges and opportunities of using lupin protein isolates in plant-based products, emphasising the need for targeted process modifications to improve gelation and exploit the thermal resilience of L. angustifolius for future food innovations.

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